Al-F Bonds Research Articles

Unraveling the structural and vibrational response of Zharchikhite to pressure (II): Insights from first principles calculations

The halide mineral Zharchikhite with the chemical formula AlF(OH)2 is studied using first-principles methods of density functional theory with the local functional VBH, generalized gradient functionals PBE, BLYP, PBEsol, with an empirical dispersion correction in the form of D3(BJ) PBE-D3, BLYP-D3, global hybrid functionals PBE0, B3LYP, PBEsol0, hybrid functionals with range separation HSE06, SC-BLYP. It is shown that all functionals overestimate O-H and underestimate O-H∙∙∙O distances. The best values are provided by SC-BLYP, B3LYP. The bulk modulus, determined from the Birch-Murnaghan equation of state, is 72.32 GPa. The axes compress unevenly and the fastest is the longest one with a modulus of 217.3 GPa. The linear compressibility modulus of the Al-F bond has a similar value and is twice as large for the Al-O bond. Under pressure, the lengths of H∙∙∙O hydrogen bonds decrease faster and, conversely, O-H bonds slowly increase. Also, at a rate of 0.289°/GPa, the monoclinic angle also increases with pressure. The spectra of infrared absorption and Raman scattering of light have been calculated, in which vibrations of Al-F and Al-O atoms were actived in the region up to 600 cm−1; above 700 cm−1 there are translational vibrations of O-H with a dominant contribution of H1 (Au symmetry) or H2 (Bu symmetry) and above 3200 cm−1 100 % hydrogen atoms. For them, with increasing pressure, the wave numbers decrease and the Grüneisen parameters change from −0.36 to −0.54. The large value of the derivative dν/dP can be used to solve the inverse problem - finding the pressure P from the known wave number ν(P).

Effective removal of fluoride and lead ions from aqueous solutions using water treatment residue and rice straw co-pyrolysis biochar composites

The coexistence of fluoride and lead in the waste water often leads to compound pollution, and finding effective and cost-effective solutions remains a significant issue. In this study, water treatment residues biochar (WTR-BC) was synthesized for the first time through the co-pyrolysis of water treatment residues and agricultural wastes to remove fluoride and lead. The results indicated that WTR-BC demonstrated remarkable efficacy in adsorbing fluoride and lead, achieving a maximum capacity of 15.2 mg/g and 83.8 mg/g, respectively. The adsorption mechanism adhered to a pseudo-second-order kinetic model, which illustrated that it was primarily driven by chemisorption. Moreover, X-ray photoelectron spectroscopy analysis indicated the formation of new complexes, F-Al and F-C with F on the WTR-BC surface, and X-ray absorption fine structure spectroscopy showed that the lead in the WTR-BC samples mainly existed as lead ferrihydrite (64.7%) and Pb(C2H3O2)2 (35.3%). These results revealed that the binding of lead and fluorides to the WTR-BC surface was attributed to Fe-O-Pb, C-O-Pb-O-C, and Al-F bond structures. The successful preparation of WTR-BC provides a new approach for the treatment of complex fluoride and lead pollution and holds promise as a novel material for environmental remediation. Furthermore, it broadens the scope of reuse possibilities for both WTR and agricultural wastes.

Development of magnetic La doped Al2O3 core-shell nanoparticle loaded hydrogel for selective recovery of fluoride from aquatic medium

The selective removal of pollutants from water bodies is regarded as a conciliation between the rapid expansion of industrial activities and need of clean water for sustainability. Fluoride is one such geogenic pollutant, and various materials have already been reported. Developing an efficient field employable material is however a challenge. Herein, we report the synthesis and competencies of strategically designed magnetic La-doped Al2O3 core-shell nanoparticle loaded polymeric nanohybrid as a benchmark fluoride sorbent. A facile synthesis strategy involved fabrication of Fe3O4 magnetic core followed by growth of La doped Al2O3 shell using sol-gel method. Doping of La2O3 into Al2O3 structure was optimised (6%), resulting in Fe3O4-Al0.94 La0.06O1.5 core-shell particles which provided exceptional fluoride affinity. The obtained magnetic Fe3O4-Al0.94La0.06O1.5 core-shell nanoparticles were then loaded (22%) into alginate to form cross-linked hydrogel beads (Fe3O4-Al0.94 La0.06 O1.5-Ca-ALG). These prepared hydrogel beads were characterised and utilized for selective recovery of fluoride under different ambient conditions. Driving forces for enhanced fluoride uptake by La doped Al2O3 were investigated and explained with the help of both experimental observation and theoretical simulation. Density functional theory calculations indicated significant expansion in the cell volume of Al2O3 due to La doping which favoured the fluoride sorption. The calculated defect formation energy for the incorporation of F into Al2O3 was found to decrease in the presence of La. XPS analysis suggested direct interaction of fluoride with Al, forming Al-F bond and breaking Al-O bond. Different vital parameters for uptake were optimised. Also, kinetics, isotherm and diffusion models were evaluated. Developed hydrogel beads attained record sorption capacity of 132.3mgg-1 for fluoride. Overall, excellent stability, no leaching of constituents, effectiveness for selective fluoride recovery from groundwater, brand it a perfect epitome of sustainable water treatment application.

Polyphenols-inspired interface modification and reduced agglomeration of solid propellants via a functionalized fluorine-containing organic substance coating layer

Interface modification of aluminum particles plays a crucial part in reducing the agglomeration during the combustion of solid propellants. In this study, the aluminum (Al) particles are pretreated with gallic acid (GA) to get Al@GA with an adhesion-favorable surface, which facilitates the further coating procedure. One new fluorine-containing organic substance is prepared, namely p-HFBA. The p-HFBA further coats the Al@GA to produce Al@GA/p-HFBA. The interface between Al@GA and the coating layer p-HFBA forms AlF bonds and induces a pre-ignition reaction with an aluminum oxide shell. Hence, the agglomerate size of propellants containing Al@GA/p-HFBA significantly decreases from 176.30 μm to 84.72 μm. The agglomerates volume is reduced by approximately 97 %. Additionally, more gaseous products investigated by TG-MS provide the Al particles with greater impetus (aerodynamic forces, Fae), enabling them to leave the burning surface and reduce the residence time. The rise in Fae and drop in adhesion force (Fad) effectively reduce the agglomeration intensity according to the skeleton layer model. This work proves that interface modification can effectively enhance the combustion performance of aluminized solid propellants.

Effect of aluminum content on plasma resistance and contamination particle generation in yttrium aluminum oxide coatings

Yttrium aluminum oxide (YAO) has garnered interest due to its ability to protect the interior parts of process chambers from the corrosive effects of fluorine-based plasma. This ability is attributed to its low melting point, which enhances the internal bonding capabilities of the atmospheric plasma spray (APS) method. We explored the resistance of YAO coatings, with varying yttrium-to-aluminum content ratios, to fluorine-based plasma. These coatings were applied using the APS method and subsequently subjected to NF3/O2 gas. Our results revealed that a higher Al content ratio resulted in a decreased number of defects within the coating, which also led to selective etching at the initial Al2O3-rich sites, primarily creating relatively weak Al-F bonds and smaller particles. In particular, selecting an appropriate yttrium-to-aluminum content ratio significantly reduced the coating defects and enhanced the etch rate compared to coatings with a higher yttrium content ratio. This improvement was attributed to the balanced presence of Al-F and Y-F bonds. Furthermore, for coatings rich with the yttrium aluminum garnet (YAG) phase, the resistance to fluorine-based plasma was primarily influenced by the thickness of the APS-Y2O3 layer. This layer, used to augment adhesion to parts, relies on the low-temperature crystallization features of YAG. The findings of this study offer valuable insights for optimizing YAG coating in dry-etching processes requiring high-density APS coatings.

Fluoride removal from coal mining water using novel polymeric aluminum modified activated carbon prepared through mechanochemical process

Defluoridation of coal mining water is of great significance for sustainable development of coal industry in western China. A novel one-step mechanochemical method was developed to prepare polymeric aluminum modified powder activated carbon (PAC) for effective fluoride removal from coal mining water. Aluminum was stably loaded on the PAC through facile solid-phase reaction between polymeric aluminum (polyaluminum chloride (PACl) or polyaluminum ferric chloride (PAFC)) and PAC (1:15 W/W). Fluoride adsorption on PACl and PAFC modified PAC (C-PACl and C-PAFC) all reached equilibrium within 5 min, at rate of 2.56 g mg-1 sec-1 and 1.31 g mg-1 sec-1 respectively. Larger increase of binding energy of Al on C-PACl (AlF bond: 76.64 eV and AlFOH bond: 77.70 eV) relative to that of Al on C-PAFC (AlF bond: 76.52 eV) explained higher fluoride uptake capacity of C-PACl. Less chloride was released from C-PACl than that from C-PAFC due to its higher proportion of covalent chlorine and lower proportion of ionic chlorine. The elements mapping and atomic composition proved the stability of Al loaded on the PAC as well as the enrichment of fluoride on both C-PACl and C-PAFC. The Bader charge, formation energy and bond length obtained from DFT computational results explained the fluoride adsorption mechanism further. The carbon emission was 7.73 kg CO2-eq/kg adsorbent prepared through mechanochemical process, which was as low as 1:82.3 to 1:8.07 × 104 compared with the ones prepared by conventional hydrothermal methods.

Effect of mechano-chemical activation with NaF on improved acid leaching of vanadium-bearing shale

Vanadium extraction from vanadium-bearing shale by direct acid leaching is currently restricted owing to the inefficient release of vanadium from mica, which consumes a large amount of acid. To overcome this limitation, this study developed a mechano-chemical activation-assisted acid-leaching process. Further, the effects of the mechano-chemical activation parameters on the vanadium-bearing shale and vanadium dissolution in an acid system were examined using the following conditions: ball-to-pulp ratio: 50 to 1, pulp density: 50%, NaF activator: 5 wt%, activation time: 30 min, sulfuric acid concentration: 12 vol%, leaching time: 6 h, leaching temperature: 95 °C. These conditions increased the vanadium leaching efficiency from 82.3% to 90.3%. The mechano-chemical activation treatment exposed the fresh muscovite surface, promoting the preferential adsorption of fluoride to form stable SiF and AlF bonds. This behavior induced a beneficial reaction surface on muscovite with a low energy barrier for subsequent leaching. During the leaching process, the weakened SiO and AlO bonds could be more easily broken by H+ attack, thus accelerating muscovite dissolution and boosting the vanadium leaching efficiency.

Estimation of the proportions of aluminum-fluorine species in NaF-AlF3 mixtures by in situ Raman spectroscopy and theoretical simulations

The quantitative distribution of different types of clusters in a binary NaF-AlF3 molten salt system with a series of cryolite ratios (CR, NaF/AlF3 molar ratio) was explored by in situ high-temperature Raman spectroscopy in conjunction with quantum chemistry (QC) ab initio calculations. Density functional theory (DFT) was used to simulate the Raman spectra of solid crystalline Na5Al3F14 (CR = 1.67) and Na3AlF6 (CR = 3.00). Aluminum-fluorine molecular model clusters containing species with typical characteristic microstructures were built in a molten NaF-AlF3 system. Deconvolution of the major stretching vibrational bands of Al-F-Al and Al-F bonds was carried out by using the Voigt function. The anionic fractions of different species present in the molten NaF-AlF3 system were quantitatively analyzed by a function of the microstructure dependent on Raman scattering cross sections (RSCS). AlF63-, AlF52-, and AlF4- were the dominant species in the aluminum-fluorine molten salt system. As the NaF in the melt increased, the content of AlF52- reached the maximum concentration when CR = 2.00. Apart from the recognized existence of both AlF4- and AlF63-, the aluminum bridging fluorine species of Al2F7- has been proven to exist in molten systems when CR ≤ 1.00. The relationship between the viscosity and various (n-3)Na+·AlFn(n-3)- species distributions has been investigated. The results showed that the contribution of different (n-3)Na+·AlFn(n-3)- to viscosity was essentially the same, and simply depended on the number of different species.

Degradation of Perfluorooctanoic Acid on Aluminum Oxide Surfaces: New Mechanisms from Ab Initio Molecular Dynamics Simulations.

Perfluorooctanoic acid (PFOA) is a part of a large group of anthropogenic, persistent, and bioaccumulative contaminants known as per- and polyfluoroalkyl substances (PFAS) that can be harmful to human health. In this work, we present the first ab initio molecular dynamics (AIMD) study of temperature-dependent degradation dynamics of PFOA on (100) and (110) surfaces of γ-Al2O3. Our results show that PFOA degradation does not occur on the pristine (100) surface, even when carried out at high temperatures. However, introducing an oxygen vacancy on the (100) surface facilitates an ultrafast (<100 fs) defluorination of C-F bonds in PFOA. We also examined degradation dynamics on the (110) surface and found that PFOA interacts strongly with Al(III) centers on the surface of γ-Al2O3, resulting in a stepwise breaking of C-F, C-C, and C-COO bonds. Most importantly, at the end of the degradation process, strong Al-F bonds are formed on the mineralized γ-Al2O3 surface, which prevents further dissociation of fluorine into the surrounding environment. Taken together, our AIMD simulations provide critical reaction mechanisms at a quantum level of detail and highlight the importance of temperature effects, defects, and surface facets for PFOA degradation on reactive surfaces, which have not been systematically explored or analyzed.

Preparation of low-cost aluminum-loaded longan shell adsorbent for fluoride removal: Experimental and modeling studies

Low-cost adsorbents for efficient defluoridation from drinking water have been an issue of great concern. Aluminum-loaded longan shell (LGS-Al) was prepared by a chemical co-precipitation method and was optimized following the conditions of longan shell of 5.12 g/200 mL, aluminum chloride concentration of 0.34 mol/L and water bath temperature of 68.3 °C. The as-prepared LGS-Al exhibited the amorphous structure, which is beneficial to developing functional groups of -Cl and -OH, and high reactivity of coordinatively unsaturated aluminum. The adsorption kinetic and isotherm data were better described by the pseudo-second-order and Sips models, respectively. The maximum adsorption capacity was 21.49 mg/g, which was higher than numerous reported biomass-based adsorbents. LGS-Al showed over 89.7% fluoride removal and limited leaching of aluminum and dissolved organic carbon in the initial pH range of 6.0–8.0. The exchange action of -OH and -Cl between fluoride and the formation of Al-F bonds were also confirmed by attenuated total reflectance fourier transform infrared (ATR-FTIR) spectra and X-ray photoelectron spectroscopy (XPS). The adsorption energy (Eads) of the Al-OH site for F- (−826.7 kJ/mol) was lower than that of Al-Cl, illustrating that Al-OH dominated the removal of F-. On the contrary, the Eads of Al-Cl for HF was lower (−121.8 kJ/mol), suggesting Al-Cl was a priority to react with HF. These findings indicated that LGS-Al could be a promising low-cost material for fluoride removal from drinking water.

Effect of CaF2 substitution by CaO on spectroscopic properties of oxyfluoride glasses

The main purpose of the present study was to investigate the optical properties and structural changes of oxyfluoride glasses containing various amounts of CaF2 (CaO). Four nominal chemical compositions, i. e., 45SiO2-20Al2O3-(30-x)CaF2-(5+x)CaO (x = 0, 5, 10, 15) (mole ratio), were chosen and glass samples were obtained by convenient melt- quenching process. On the basis of DSC curves, the peak temperature of CaF2 crystallization was increased from 673 °C to 805 °C by replacing CaF2 with CaO. This result was related to the reduction of the number of non-bridging fluorines (NBFs) and non-bridging oxygens (NBOs) in presence of higher CaO amounts. Increment of density (from 2.74 to 2.92 g/cm3) and decrement of molar volume (from 27.24 to 24.48 cm3) of samples approved this claim to some extent. In addition, according to XRD patterns, the size of CaF2 nanocrystals got smaller (from 29.21 to 10.13 nm) by increasing the quantity of CaO. This was ascribed to the difficulty of the penetration of ions through the diffusion barriers around fluoride nanocrystals in glasses with more continuous structure. Fluorine loss measurements and Structural studies showed that more CaO addition decreased NBFs and NBOs and more Al-F bonds were increased compare to Si-O-Al bonds. UV–vis spectra were used to determine the optical properties comprising Fermi energy level, optical band gaps and Urbach energy. Calculations of Fermi energy and optical band gaps demonstrated the reduction of weak and dangling bonds in samples with more CaO. Moreover, Urbach energy, the indicator of the disorderliness of the network, was decreased from 0.275 to 0.245 eV. The sample containing 25 and 10 mol ratio of CaF2 and CaO respectively, showed higher transmittance in IR region and its cut-off was red-shifted to higher wavelengths in comparison to other glasses.

Paradoxical Filler Size Effect on Composite Wear: Filler–Matrix Interaction and Its Tribochemical Consequences

The addition of 0.2–5% nanoscale (40–80 nm) α-phase or microscale (40 μm) γ-phase Al2O3 particles in PTFE effectively reduce the matrix wear rate by 99.99%, whereas microscale (> 0.5 μm) α-Al2O3 or nanoscale (40–80 nm) γ-Al2O3 only reduce PTFE wear by ~ 90% under identical loading, dispersion and testing conditions. This paradoxical material system best illustrates the complexity of tribology and the importance of filler–matrix interactions at small scales. We studied the independent effect of the Al2O3/PTFE interface area and alumina structure by systematically varying the particle size over two orders of magnitude for both α- and γ-Al2O3/PTFE composites. Detailed characterizations of filler size, surface area and tribofilm’s chemical composition were conducted. The results found: (1) DLS median particle sizes conformed reasonably to vendor reported values and percentages of microscale filler aggregates correlated weakly with wear rates, (2) electron microscopy of the as-worn composite surface suggested a strong relation between the characteristic size of ‘unreinforced’ polymer domain and composite wear rate, (3) third bodies (i.e., transfer films, debris) played an important role in counterface abrasion, (4) wear rate correlated strongly with filler’s specific surface area and ultralow wear was only maintained ~ 0.3–10 m2/g nominal specific filler–matrix area values, (5) ultralow wear coincided with perfluorinated carboxylic salt rich tribofilms which supported a previously proposed wear reduction mechanism that mechanochemically degraded PTFE chelate with alumina and cause crosslinked and wear-resistant tribofilms, (6) tribofilm Al-F bond signal increased with filler surface area and high wear coincided with excessive tribofilm Al-F signal for γ-Al2O3/PTFE systems. Based on these results and literature hypothesis, we proposed that (1) the 1 μm α-Al2O3 provided the least filler–matrix interface and largest unreinforced polymer domain in PTFE, which lead to the least crosslinked and compartmentalized tribofilms; (2) in γ-alumina filled composites, Al-F bond forms as a product of mechanochemically degraded PTFE but also blocks chelation between the degraded PTFE and alumina fillers, (3) the 20 nm γ-Al2O3 provided the most filler–matrix interface which leads to excessive aluminum fluoride that blocked the filler–matrix chelation, prevented the tribofilm crosslinking and lead to high wear rates. This hypothesis was additionally supported by small molecule experiments in this study. However, this study provides no direct insight into how sensitive the filler–matrix tribochemical interaction is to filler phase or aggregate strength (strong, weak or fully dense).

Comprehensive investigation on CF4/O2-plasma treating the interfaces of stacked gate dielectric in MoS2 transistors

CF4/O2-plasma is used to treat the interfaces of gate stacks (MoS2/Al2O3/ ZrO2/p+-Si), and its effects on the electrical performance of the few-layered MoS2 transistors have been investigated. Experimental results show that excellent electrical properties have been achieved for the MoS2 transistors with CF4/O2-plasma treated stacked gate dielectric, especially for the F-treated ZrO2 device: high mobility of 53.7 cm2/Vs, small subthreshold swing of 117 mV/dec and high on/off ratio of 2.7 × 107. The involved main mechanisms lie in the facts that: (i) the MoS2/dielectric interface is improved through the F- passivation on the oxygen vacancies in the dielectric, decreasing the dangling bonds and surface roughness of the dielectric; (ii) the dielectric constant (k) of the gate stack dielectric is increased because of suppressed growth of low-k Zr-silicate interlayer and improved dielectric quality by forming ZrF and AlF bonds; (iii) CF4/O2-plasma treatment on the ZrO2 surface can result in F incorporation in both ZrO2 and Al2O3, thus giving the best passivation effects on oxygen vacancies and oxide traps near/at the MoS2/dielectric interface. These results indicate that CF4/O2-plasma treatment is an effective way for improving the bulk and interface qualities of gate dielectrics and thus the electrical properties of MoS2 FETs.

Electrical Characterization of Charge Polarity in AlF₃ Anti-Reflection Layers for Complementary Metal Oxide Semiconductor Image Sensors.

In this study, the charge polarity of aluminum fluoride (AlF3) as a function of varying thickness (tAlF3 = 20, 35, 50, 65, and 80 nm) was discussed. AlF3 films were deposited onto p-Si wafers via electron beam sputtering. Thickness dependent charge polarity and reliability issues under bias-temperature stress conditions were identified using a capacitance-voltage (C-V) characterization method. AlF3 was found to possess negative fixed charges, leading to a C-V curve shift toward the positive gate bias direction as tAlF3 was increased up to 50 nm. On the contrary, the C-V characteristics were dominantly affected by the positive charges of mobile ions and/or fluorine vacancies when tAlF3 was increased to more than 50 nm. Additionally, negative bias temperature stress (1 MV/cm, 473 K for 10 mins) increased insulator trapped charges and decreased interface traps in 20 nm thick AlF3 films. These results could be attributed to positively charged fluorine vacancies introduced by broken Al-F bonds within AlF3 films and the passivation of Si dangling bonds due to broken fluorine ions at the interface, respectively. It was believed that 20 nm thick AlF3 films sufficiently attracted holes from the Si substrate, forming a hole accumulation layer on the surface due to total charge polarity of the AlF3 dielectric being entirely governed by negative fixed charges as the thickness of AlF3 decreased. Based on these results, AlF3 films are proposed for use as an anti-reflection layer to replace HfO2 in CMOS image sensors.

Tailored white light emission in Eu3+/Dy3+ doped tellurite glass phosphors containing Al3+ ions

Tellurite glass systems modified by addition of aluminum fluoride AlF3 have been successfully synthesized as host matrices for optically active rare earth ions RE3+ (RE3+ = Eu3+, Dy3+). Samples with different Eu3+ to Dy3+ molar ratio have been studied in order to determine possibility of white light emission via UV excitation. Structural investigations confirmed amorphous character of materials whereas spectroscopic studies brought more insight into glass network's nature. FTIR results shown presence of two features related to tellurite glass matrix (in 490–935 cm−1 spectral region) and another one (940-1250 cm−1) due to aluminum addition. Especially, Al-O and Te-O-Al bonds of AlO4 tetrahedrons have been found. AlO4 units are considered as glass formers that improve network's strength and thermal resistivity against devitrification. Based on XPS studies of Al3+ photoelectron band the existence of Al-O and also Al-F bonds have been examined. Moreover, signals originating from Eu3+ and Dy3+ have been found confirming their valence state. Luminescence results revealed possibility of simultaneous UV excitation of Eu3+ and Dy3+ ions. Excitation with λexc = 390 and 393 nm resulted in white light generation starting from warm white to neutral and cool white depending on Eu3+ concentration and used excitation wavelength. Additionally, increase of decay lifetime of Eu3+ induced by Al3+ presence have been revealed based on luminescence decay analysis. Thus, tellurite glass systems modified by AlF3 and doped with Eu3+/Dy3+ may be considered as promising candidates for white light emitting sources.

Elastic, electronic structure, and optical properties of orthorhombic Na3AlF6: a first-principles study

First-principles investigation of elastic, electronic, and optical properties of orthorhombic Na3AlF6 has been carried out by DFT using plane-wave pseudo-potentials within the LDA and GGA. Calculated lattice parameters agree well with experimental results. From calculated elastic constants, Na3AlF6 is a mechanically stable anisotropic and behaves in a ductile manner. Electronic structure analysis indicates that Na3AlF6 behaves as an insulator with a direct band gap of 6.065 eV in LDA and 5.868–5.949 eV in GGA. DOS, population analysis, and charge densities difference indicate that Al-F bonds are mainly ionic as well as partially covalent due to the hybridization of F-2p and Al-3s (3p) states. Moreover, the imaginary part of calculated dielectric function e2(ω) shows three prominent peaks due to the inter band transitions F 2p states→Na 3s states. From calculated e (ω), other optical properties such as reflectivity and refractive index are also obtained up to the photon energy range of 40 eV.

An Aluminum Fluoride Complex with an Appended Ammonium Salt as an Exceptionally Active Cooperative Catalyst for the Asymmetric Carboxycyanation of Aldehydes.

Al-F bonds are among the most stable σ bonds known, exhibiting an even higher bond energy than Si-F bonds. Despite a stability advantage and a potentially high Lewis acidity of Al-F complexes, they have not been described as structurally defined catalysts for enantioselective reactions. We show that Al-F salen complexes with appended ammonium moieties give exceptional catalytic activity in asymmetric carboxycyanations. In addition to aromatic aldehydes, enal and aliphatic substrates are well accepted. Turnover numbers up to around 104 were achieved, whereas with previous catalysts 101 -102 turnovers were typically attained. In contrast to Al-Me and Al-Cl salen complexes, the analogous Al-F species are remarkably stable towards air, water, and heat, and can be recovered unchanged after catalysis. They possess a considerably increased Lewis acidity as shown by DFT calculations.

The physical mechanism on the threshold voltage temperature stability improvement for GaN HEMTs with pre-fluorination argon treatment

In this paper, a normally-off AlGaN/GaN MIS-HEMT with improved threshold voltage (VTH) thermal stability is reported with investigations on its physical mechanism. The normally-off operation of the device is achieved from novel short argon plasma treatment (APT) prior to the fluorine plasma treatment (FPT) on Al2O3 gate dielectrics. For the MIS-HEMT with FPT only, its VTH drops from 4.2 V at room temperature to 0.5 V at 200 °C. Alternatively, for the device with APT-then-FPT process, its VTH can retain at 2.5 V at 200 °C due to the increased amount of deep-level traps that do not emit electrons at 200 °C. This thermally stable VTH makes this device suitable for high power applications. The depth profile of the F atoms in Al2O3, measured by the secondary ion mass spectroscopy, reveals a significant increase in the F concentration when APT is conducted prior to FPT. The X-ray photoelectron spectroscopy (XPS) analysis on the plasma-treated Al2O3 surfaces observes higher composition of Al-F bonds if APT was applied before FPT. The enhanced breaking of Al-O bonds due to Ar bombardment assisted in the increased incorporation of F radicals at the surface during the subsequent FPT process. The Schrödinger equation of Al2OxFy cells, with the same Al-F compositions as obtained from XPS, was solved by Gaussian 09 molecular simulations to extract electron state distribution as a function of energy. The simulation results show creation of the deeper trap states in the Al2O3 bandgap when APT is used before FPT. Finally, the trap distribution extracted from the simulations is verified by the gate-stress experimental characterization to confirm the physical mechanism described.

Molecular dynamics investigation on structural and transport properties of Na3AlF6–Al2O3 molten salt

To study the effect of Al2O3 concentration on the local structure and transport properties of Na3AlF6–Al2O3 molten salt, molecular dynamics (MD) simulations with the Coulomb–Buckingham potential model were carried out. MD simulation results show [AlF4]−, [AlF5]2− and [AlF6]3− groups coexist in Na3AlF6–Al2O3 molten salt and [AlF6]3− ions are the dominant species at low Al2O3 concentration of 1–2wt%. The percentages of [AlF4]−, [AlF5]2− and [AlF6]3− groups are approximately equal at 3wt% Al2O3 and [AlF4]− ions become the major species at 4wt% Al2O3. O atoms mainly exist in the form of bridge Ob up to 90%, worsening the fluidity and transport properties of Na3AlF6–Al2O3 molten salt. According to quantum chemical calculations, AlF and AlO bonds in the [Al2OF6]2− groups have ionic characters as well as partial covalent characters due to the hybridization of F, O-2p and Al-3s (3p) orbitals, while NaF and FF bonds are ionic. The orders of particle's diffusion ability follow as Na+>F−>Al3+>O2–. In addition, Al2O3 acts as bridges connecting ionic structure networks and more Al2O3 can increase the polymerization degree of the local structure in Na3AlF6–Al2O3 molten salt, and the viscosity increase and ionic conductivity reduce accordingly.

Theoretical investigation on local structure and transport properties of NaF[sbnd]AlF3 molten salts under electric field environment

The effect of electric field and molecular ratio CR (NaF/AlF3) on basic structure and transport properties of NaFAlF3 molten salts were investigated by molecular dynamics simulations with the Buckingham potential model. The [AlF6]3− groups are the dominant specie in NaFAlF3 molten salts at CR ≥ 2.6, and followed by the [AlF5]2− groups, while CR ≤ 2.4, [AlF5]2− groups are the protagonists up to 40%. In NaFAlF3 system, with the increase of CR, the proportion of Fb decreases slightly and the percentage of Ff increases dramatically. The AlF bonds have ionic characters as well as partial covalently characters due to the hybridization of F-2p and Al-3s, 3p orbitals. The order of ion diffusion ability follows as Na+ > F− > Al3+. Adding more NaF can break some F bridges of structure networks and decrease the polymerization degree of NaFAlF3 molten salts, the viscosity reduces and ionic conductivity increases as a consequence. The calculated results of ionic conductivity are in agreement with the experimental results. Electric field has no significant impact on the local structure characters, while transport properties are not. The change of CR (NaF/AlF3) can significantly affect these characters of both the structure and transport.